1. Field of the Invention
This invention relates to a bearing equipped with a lubricating layer and a method for providing a lubricating layer to a bearing.
2. Description of the Related Art
Conventional bearings equipped with a lubricating layer include the bearings disclosed in JP-A-2004-257491 and JP-A-H11-153144. The conventional bearings will be described with reference to FIG. 7 to FIG. 10.
The bearing 1 illustrated in FIG. 7 comprises a tubular retainer 2 fitted in an outer ring 3. The retainer 2 has a plurality of roller-retaining holes formed therein, each of which extends in the axis direction thereof, and arranged at regular intervals on the circumference. Needle rollers 4 are rotatably retained in the respective roller-retaining holes.
As shown in FIG. 8, a shaft 5 is inserted in the above bearing 1, in which the rollers 4 are interposed between the outer ring 3 and the shaft 5, and the rollers 4 rotate between the outer ring 3 and the shaft 5 so as to make their relative rotation smooth.
A lubricating layer G made of a solid lubricant is provided uniformly in the bearing 1. Accordingly, the outer periphery faces of the rollers 4 are always covered with oil or the like impregnated in the lubricating layer G. As a result, the relative rotation between the rollers 4 and the outer ring 3 and the relative rotation between the rollers 4 and the shaft 5 are made smooth and frictional heat is not generated in the contact portions between the rollers 4 and them. Reference letter S in FIG. 8 denotes an inscribed circle. The inscribed circle here means a virtual circle on which the shaft 5 comes into contact with all the rollers 4 held in the retainer 2. However, two circles in contact with all the rollers 4 are formed on the inside and the outside of the rollers 4, and the inscribed circle in this case means the circle formed on the inside of the rollers 4, which is approximately aligned with the outer periphery of the shaft 5 in FIG. 8.
Another bearing 6 illustrated in FIG. 9 has a slight space x extending from the outer periphery of the shaft 5 toward the retainer 2 in which the lubricating layer G is not formed. Such provision of a slight area without the lubricating layer G prevents the lubricating layer G from making its way into the gap between the roller 4 and the rolling face of the shaft 5.
Next, the method of providing the lubricating layer G in each of the bearings 1, 6 will be described.
First, the retainer 2 is inserted in the outer ring 3 and the rollers 4 are inserted in the roller-retaining holes 2a from the inside of the retainer 2 to form the bearing 1. Then, the bearing 1 is inserted in a forming die 7 having an internal diameter approximately equal to the outer diameter of the outer ring 3, and then an aluminum-made shaft member 8 having a shaft diameter equal to or slightly smaller than the roller-set-bore diameter S1 of the plurality of rollers 4 in the bearing 1 is inserted in the bearing 1 (the inscribed circle S). Here, the roller-set-bore diameter S1 means the diameter of the inscribed circle S formed by the plurality of rollers 4 held in the retainer 2.
At this stage, a fluid lubricant 9 including a thermoplastic resin is fed into the forming die 7 so as to fill the empty space created in the bearing 1 with the fluid lubricant 9. Then, the inside of the forming die 7 is hermetically sealed and reduced in pressure or remove any air bubbles included in the fluid lubricant 9. Then, the entire forming die 7 is heated to about 150° C., thermally setting the thermoplastic resin included in the fluid lubricant 9.
With this method, as is seen from FIG. 7 and FIG. 8, the lubricating layer G can be uniformly formed in each area between the outer ring 3 and the retainer 2, between the roller-retaining holes 2a and the rollers 4 and between the inscribed circle S of the rollers 4 (the shaft 5) and the retainer 2, except for the contact portions between the outer ring 3 and the rollers 4 and the slight contact portions between the shaft 5 and the rollers 4.
In the process of forming the lubricating layer G in the bearing 6, a spacer (not shown) is attached to the inside of the bearing 6, whereby a space x in which the lubricating layer G is not formed is created in a range from the inscribed circle S of the rollers 4 in correspondence with the thickness of the spacer. Accordingly, as is seen from FIG. 9, the lubricating layer G can be uniformly formed in the bearing 6, except for the contact portions between the outer ring 3 and the rollers 4 and the slight space x extending from the inscribed circle S toward the retainer 2. In other words, it is possible to positively ensure the existence of a space x corresponding to the thickness of the spacer.
In the bearing 1 in the initial state, the lubricating layer G is provided uniformly on approximately the full periphery of each of the rollers 4, except for a slight area including the contact portions between the outer ring 3 and the rollers 4 and the contact portions between the shaft 5 and the rollers 4. That is, in the initial state the lubricating layer G is in close contact with approximately the full periphery of the roller 4. In the initial state of close contact between the lubricating layer G and approximately the full periphery of the roller 4, the close-contact force of the lubricating layer G on the roller 4 is large. Therefore, when the rollers 4 roll in the initial state, the rollers 4 require a large torque because of the effect of the close-contact force. Because of this requirement, when the outer ring 3 is rotated at a small load in this initial state, the rollers 4 do not roll smoothly, so that the rollers 4 and tree outer ring 3 come into sliding contact with each other. Such sliding contact between the rollers 4 and the outer ring 3 generates frictional heat, possibly causing seizing up between the rollers and the outer ring 3 and/or plastic deformation of the contact portion. To avoid this, in the initial state when the bearing 1 is used, the imposition of a load of 1% or more of the basic dynamic load rating is indispensable for providing the normal rotation.
On the other hand, in the bearing 6 with the space x created as described earlier, the area of the lubricating layer G which is in contact with the outer periphery surface of the rollers 4 is reduced by the space x as compared with the case of the bearing 1. For this reason, even in the initial state, the bearing 6 enables the rotation of the rollers 4 with a small torque. As a result, the minimum bearing load imposed on the bearing 6, which is required in the initial state, is smaller than in the case of the bearing 1.
However, for the bearing 6, a spacer is required for creating the space x when the lubricating layer G is formed. In the case of the bearing 6, a plurality of spacers is arranged in each bearing 6 in the process of forming the lubricating layer G. This gives rise to an increase in the manufacturing cost.
Further, the spacers are small items and the required number of spacers for each bearing corresponds to the number of intervals between the adjacent rollers 4. Therefore, as well as the process of arranging the spacers being difficult, the number of man-hours is increased, increasing the manufacturing cost.